Cheapest priced copy (Commander 2020): $0.35 at Card Kingdom, near mint, their price list as of .
Whiplash Trap 
Instant — Trap · Common · Zendikar · #77
If an opponent had two or more creatures enter the battlefield under their control this turn, you may pay {U} rather than pay this spell's mana cost.
Return two target creatures to their owners' hands.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
5 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2020 | 127 | Common | $0.35 | — |
| Duel Decks: Speed vs. Cunning | 70 | Common | $0.35 | — |
| Planechase | 16 | Common | $0.35 | — |
| The List | DDN-70 | Common | $0.35 | — |
| Zendikar | 77 | Common | $0.35 | $0.79 |
5 rulings from Wizards of the Coast
- 1 October 2009You must target two creatures as you cast Whiplash Trap. If you can't (because just one creature is on the battlefield, perhaps), you can't cast the spell.
- 1 October 2009Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
- 1 October 2009You can target any two creatures, not just ones that entered this turn.
- 1 October 2009Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.
- 1 October 2009You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.